Interpretive Summary: The increasing atmospheric CO2 concentration is likely to cause partial stomatal closure in the leaves of plants growing in the future, and such partial closure would reduce water loss from individual leaves. Several experiments have been conducted over the last 15 years using free-air CO2 enrichment (FACE) in open-field-grown crops in which measurements related to water and energy exchange were made. They show that canopy temperatures likely will increase about 0.5 degreesC (0.9 degrees F) and that plant water requirements will be reduced from zero to 20%, depending on the crop and conditions. The reduction in water use would lead to increases in soil water content and improvements in the internal water status of plants, thereby increasing growth under water-limited conditions. This research should benefit growers and consumers of food and fiber, as well as water resource planners.

Technical Abstract:
Because elevated concentrations of CO2 cause partial stomatal closure, transpiration from plant leaves is reduced, which has many ramifications for plant water relations. First, the reduction in leaf transpiration reduces evaporative cooling with a consequential rise in canopy temperatures. Increases of 0.3 to 1.7 degrees C at CO2 concentrations of 550 umol mol-1 (200 umol mol-01 above present-day ambient) have been observed depending on species and conditions. Such canopy temperature changes are likely to cause shifts in the optimum geographic climate areas for growth of crops and other species.
Second, the reduction in transpiration per unit of leaf area with elevated CO2 generally leads to a reduction in evapotranspiration (ET) per unit of land rea. However, the magnitude of such water conservation at elevated CO2 varies with the degree of stimulation of plant growth and the degree of partial stomatal closure. Observed reductions in ET have ranged from near zero for cotton, a woody C3 species with large growth stimulation, to about 16% for sorghum, a C4 grass with little growth stimulation. In the absence of global warming, such water conservation will reduce the water requirements of irrigated regions, and with global warming, it will help to keep the requirements from rising as much as the warming alone would cause.
Third, the reductions in ET with elevated CO2 will also lead to increases in soil moisture content with consequent effects on numerous soil physical, chemical, and biological processes that are influenced by soil moisture content, such as leaching, mineralization, and soil respiration.
Lastly, the reductions in ET and consequent increases in soil moisture can lead to improvements in plant water relations, such as higher plant water potentials. Water conservation with growth in elevated CO2 can enable plants to maintain growth longer into drought cycles.